1. Field of the Invention
The invention relates to a receiver for a digital radio signal, particularly wherein the filter coefficient is changeable by a control circuit which generates a first value for the filter coefficient (APK) to calculate the real signal component and thereafter generates a second value for the imaginary signal component, whereby with the switching over of the first and second values for the filter coefficient (APK) a simultaneous downsampling of the digital signal is performed.
2. Description of the Prior Art
Presently, receivers for radio transmissions generally operate using analog VHF transmission. However, in order to improve the transmission quality, digital processes are frequently used, such as digital satellite radio (DSR). Thereby, a transmission signal is transmitted from the transmitter via satellite to the receiver. In order not to equip every single receiver with an antenna and a first low-noise mixer for the satellite radio, which are relatively large and expensive, a DSR-signal may be fed from a central terrestrial receiver also over broadband cable networks to the receiver.
However, for the mobile receiving in motor vehicles or portable devices, this is not practical. In order to provide transmission for mobile reception which corresponds to the quality standard offered by the digital technology (e.g., digital compact cassette, compact disc), digital audio broadcasting (DAB) was developed wherein individual carrier frequencies are simultaneously emitted with an equidistant frequency distance in a frequency range having the bandwidth B. Thus, signal distortions which result from frequency selective field strength fluctuations and multipath reception are prevented. The individual carriers are each modulated with a part of the digital data, whereby the modulation contents of the individual carriers are identical for all transmission stations. When the operation is time multiplexed, data of different programs can be transmitted in a timely sequence within the data package, so that for change in program in the receiver, no change of the tuning frequencies is required, rather the sequentially associated decoding of the data packages is changed.
A digital receiver is used to achieve the best possible reception quality of a digital transmission signal. Such a digital receiver is known from DE-34 28 318 A1. In this receiver, the analog reception signal is mixed down into an intermediate frequency signal by a mixer, will be band-limited with an analog bandpass filter, sampled, analog-to-digital converted and multiplied with complex signal of an oscillator in at least one digital signal processor. In this manner, the quadrature components of the reception signal are generated in the baseband.
The disadvantage in this receiver is the extensive processing of the analog-to-digital converted signal. At least one digital signal processor is required for performing a multiplication with a complex oscillator signal.
A digital demodulator for frequency modulated signals is disclosed in EP-0 201 758 B1, which enables compensation of multipath reception. In this demodulator, the reception signal is also first mixed down into an intermediate frequency signal, band-limited and analog-to-digital converted. Thereafter, the digital signal is fed to a cascading equalizer which performs a reduction of the reflections contained in the input signal.
This demodulator is disadvantageous in that it requires a complex circuit and corresponding engineering effort, particularly in view of the cascading equalizers.
A circuit is disclosed in the IEEE-publication Vol. AES-20, No. 6, November 1984, pages 821-824, entitled "A Simple Method of Sampling In-phase and Quadrature Components", which mixes down the reception signal, band limits the same, samples and performs analog-to-digital conversion. The digital signal is downsampled and fed to two parallel switched all-pass filters, of which the one emits the real signal component and the other the imaginary signal component of the input signal.
This circuit is disadvantageous due to its very high sampling rate, which requires a high processing speed in the analog-to-digital converter. Furthermore, the two fifth order all-pass filters require a complex switch.